dccp: Print deprecation notice.

DCCP was marked as Orphan in the MAINTAINERS entry 2 years ago in commit
054c4610 ("MAINTAINERS: dccp: move Gerrit Renker to CREDITS").  It says
we haven't heard from the maintainer for five years, so DCCP is not well
maintained for 7 years now.

Recently DCCP only receives updates for bugs, and major distros disable it
by default.

Removing DCCP would allow for better organisation of TCP fields to reduce
the number of cache lines hit in the fast path.

Let's add a deprecation notice when DCCP socket is created and schedule its
removal to 2025.
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/dccp/proto.c

@@ -191,6 +191,9 @@ int dccp_init_sock(struct sock *sk, const __u8 ctl_sock_initialized)
 	struct dccp_sock *dp = dccp_sk(sk);
 	struct inet_connection_sock *icsk = inet_csk(sk);
 
+	pr_warn_once("DCCP is deprecated and scheduled to be removed in 2025, "
+		     "please contact the netdev mailing list\n");
+
 	icsk->icsk_rto		= DCCP_TIMEOUT_INIT;
 	icsk->icsk_syn_retries	= sysctl_dccp_request_retries;
 	sk->sk_state		= DCCP_CLOSED;